1. Field of the Invention
This invention relates to gas-turbine engine combustion systems, and in particular to combustion systems in which combustion chambers are interconnected by crossfire tubes for ignition purposes.
2. Description of the Related Art
In a typical industrial gas-turbine engine, a number of combustion chambers (hereinafter referred to as combustors) are arranged in parallel around the engine to receive the pressurized air flow from the compressor stage as oxidant for gaseous or liquid fuel which is burnt therein. For example, there may be six to eight combustors equiangularly spaced around the engine's centerline at a given radial distance therefrom. To avoid the need for igniters in every combustor to initiate combustion on start-up, it has become common practice to interconnect the combustors with tubes, called crossfire tubes, which are adapted to pass a flame from an ignited combustor to another combustor. A problem that has been experienced with this type of arrangement is that of the crossfire tubes or the combustors becoming damaged by the flow of hot gases during normal running after start-up. One way of reducing this problem is disclosed and claimed in our European Patent No. 0 503 018. In this arrangement, air is introduced into the crossfire tube in such a manner as to be constrained to flow over the inner surface of the crossfire tube adjacent to its connection with the combustor, thereby cooling the crossfire tube without adversely affecting the cross-lighting performance, and so extending its working life.
Although this arrangement has proved to be a significant improvement over earlier crossfire tube designs, it has been found in practice that there remains a possibility of overheating of the combustor wall adjacent to the position where the crossfire tube enters it.
U.S. Pat. No. 5,001,896 discloses a crossfire tube assembly for interconnecting combustors, in which a double-walled crossfire tube is used, the outer wall being perforated to admit cooling air into the space between the walls, and the inner wall also being provided with apertures to bleed some air into the gas flow within the crossfire tube. The outer wall fits into an annular flange projecting through the combustor wall and inwardly into the combustor, while the inner wall of the crossfire tube projects beyond its outer wall into the flange. Although this arrangement improves cooling of the tube, there is still a problem with localized heating of the inwardly directed flange, as well as the combustor wall surrounding it, and the inner wall of the interconnecting tube where it projects into the flange. In extreme conditions, this localized heating might cause failure of these components, resulting in fragments of metal being propelled into the turbine, possibly in turn causing its failure. While the risks of such a major failure are very low, the likelihood of early failure of the combustor through overheating around the flange is considerably higher.
The present invention seeks to avoid these problems and therefore to improve life expectancy of the combustion system.